Magnetic field sensors (e.g., rotation detectors) for detecting ferromagnetic articles and/or magnetic articles are known. The magnetic field associated with the ferromagnetic article or magnetic article is detected by a magnetic field sensing element, such as a Hall element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field. In some arrangements, the magnetic field signal is an electrical signal.
The magnetic field sensor processes the magnetic field signal to generate an output signal that, in some arrangements, changes state each time the magnetic field signal crosses thresholds, either near to peaks (positive and/or negative peaks) or near to some other level, for example, zero crossings of the magnetic field signal. Therefore, the output signal has an edge rate or period indicative of a speed of rotation of the ferromagnetic (e.g., ferrous) or magnetic object, for example, a gear or a ring magnet (either of which may or may not be ferrous).
One application for a magnetic field sensor is to detect the approach and retreat of each tooth of a rotating ferromagnetic gear, either a hard magnetic gear or a soft ferromagnetic gear. In some arrangements, the gear is disposed proximate to a stationary magnet, which can form a part of a magnetic field sensor, and the magnetic field sensor is responsive to perturbations of a magnetic field as the gear rotates. Such arrangements are also referred to as proximity sensors or motion sensors. In other arrangements, a ring magnet having magnetic regions (permanent or hard magnetic material) with alternating polarity is coupled to the ferromagnetic gear or is used by itself and the magnetic field sensor is responsive to approach and retreat of the magnetic regions of the ring magnet. In the case of sensed rotation, the arrangements can be referred to as rotation sensors.
In one type of magnetic field sensor, sometimes referred to as a peak-to-peak percentage detector (or, more simply, threshold detector), one or more threshold levels are equal to respective percentages of the peak-to-peak magnetic field signal. One such threshold detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold,” issued Jun. 29, 1999, and assigned to the assignee of the present invention.
Another type of magnetic field sensor, sometimes referred to as a slope-activated detector (or peak-referenced detector, or, more simply, a peak detector), is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” issued Jun. 18, 2000, and also assigned to the assignee of the present invention. In the peak detector, the threshold levels differ from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of magnetic field sensor, the output signal changes state when the magnetic field signal comes away from a peak or valley of the magnetic field signal by the predetermined amount.
It should be understood that the above-described threshold detector and the above-described peak detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal. The threshold detector and the peak detector, however, each use the detected peaks in different ways.
In order to accurately detect the positive and negative peaks of a magnetic field signal, the magnetic field sensor is capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
The magnetic field associated with the ferromagnetic object and the resulting magnetic field signal are proportional to the distance between the ferromagnetic object, for example the rotating ferromagnetic gear, and the magnetic field sensing element(s), for example, the Hall elements, used in the proximity detector. This distance is referred to herein as an “air gap.” As the air gap increases, the magnetic field sensing elements tend to experience a smaller magnetic field from the rotating ferromagnetic gear, and therefore smaller changes in the magnetic field generated by passing teeth of the rotating ferromagnetic gear.
Proximity detectors have been used in systems in which the ferromagnetic object (e.g., the rotating ferromagnetic gear) not only rotates, but also vibrates. For the ferromagnetic gear capable of rotation about an axis of rotation in normal operation, the vibration can have at least two vibration components. A first vibration component corresponds to a “rotational vibration,” for which the ferromagnetic gear vibrates back and forth about its axis of rotation. A second vibration component corresponds to “translational vibration,” for which the above-described air gap dimension vibrates. The rotational vibration and the translational vibration can occur even when the ferromagnetic gear is not otherwise rotating in normal operation. Both the first and the second vibration components, separately or in combination, can generate an output signal from the proximity detector that indicates rotation of the ferromagnetic gear even when the ferromagnetic gear is not rotating in normal operation.
Proximity detectors adapted to detect and to be responsive to rotational vibration and translational vibration are described, for example, in U.S. Pat. No. 7,365,530, issued Apr. 29, 2008, U.S. Pat. No. 7,592,801, issued Sep. 22, 2009, U.S. Pat. No. 7,622,914, issued Nov. 24, 2009, U.S. Pat. No. 7,253,614, issued Aug. 7, 2007, and U.S. patent application Ser. No. 12/338,048, filed Dec. 18, 2008, each of which are assigned to the assignee of the present invention.
Proximity detectors have been applied to automobile antilock brake systems (ABS) to determine rotational speed of automobile wheels. Proximity detectors have also been applied to automobile transmissions to determine rotating speed of transmission gears in order to shift the transmission at predetermined shift points and to perform other automobile system functions.
It will be understood that many mechanical assemblies have size and position manufacturing tolerances. For example, when the proximity detector is used in an assembly, the air gap can have manufacturing tolerances that result in variation in magnetic field sensed by the magnetic field sensing elements used in the proximity detector when the ferromagnetic object is rotating in normal operation and a corresponding variation a magnitude of the magnetic field signal. It will also be understood that the air gap can change over time as wear occurs in the mechanical assembly.
Due to noise (electrical or vibrational) the motion sensor may not accurately position edges of an output signal.
For either the peak detector or for the threshold detector, it will be understood that a comparator used to generate a final two state output signal indicative of a speed of rotation of an object are influenced by electrical noise. In particular, where a magnetic field signal and window thresholds are presented to input terminals of a comparator, if the window thresholds are below the noise level, false state changes at the output of the comparator can occur. As described above, for a threshold type detector, the window thresholds are fixed percentages of a peak-to-peak magnitude of the magnetic field signal. Thus, for a smaller magnetic field signal, the window threshold can be within the noise level.
Thus, it would be desirable to have a threshold detector for which calculated thresholds are not within a noise level of a magnetic field sensor.